Planets constants

55 values, each with its units, its uncertainty, and where it came from.

Astronomical (106)Chemistry (55)Electromagnetic (31)Material Properties (69)Mathematical (16)Thermodynamic (37)Universal & Atomic (47)

All 361 constants →

A E J M N P S U V

Axial Tilt of Mars

\varepsilon_{\mathrm{Mars}} = 0.4396484\ \text{rad}

radObliquity of Mars, 25.19° — a near-twin of Earth's tilt today, but chaotic over millions of years for want of a stabilising moon.

Axial Tilt of Uranus

\varepsilon_{\mathrm{Uranus}} = 1.7064084\ \text{rad}

radObliquity of Uranus, 97.77° — the planet rolls around its orbit on its side, poles sunward, unlike anything else in the solar system.

Equatorial Radius of Jupiter measured

R_{\mathrm{J}} = 71,492,000\ \text{m}

mJupiter's equatorial radius at the 1-bar level, 71 492 km; the polar radius is 66 854 km, a 6.5 per cent flattening from fast rotation.

Equatorial Radius of Neptune measured

R_{\mathrm{Neptune}} = 24,764,000\ \text{m}

mNeptune's equatorial radius at the 1-bar level, 24 764 km — slightly smaller than Uranus while noticeably heavier, the denser twin.

Equatorial Radius of Saturn measured

R_{\mathrm{Saturn}} = 60,268,000\ \text{m}

mSaturn's equatorial radius at the 1-bar level, 60 268 km — nine and a half Earths across, not counting the quarter-million-kilometre rings.

Equatorial Radius of Uranus measured

R_{\mathrm{Uranus}} = 25,559,000\ \text{m}

mUranus's equatorial radius at the 1-bar level, 25 559 km — four Earths across, measured almost entirely from one 1986 flyby.

Escape Velocity of Jupiter

v_{\mathrm{esc},\mathrm{J}} = 59,500\ \text{m/s}

m/sSpeed needed to escape Jupiter from the 1-bar level, 59.5 km/s — a well so deep the planet has kept every gas since it formed.

Escape Velocity of Mars

v_{\mathrm{esc},\mathrm{Mars}} = 5,030\ \text{m/s}

m/sSpeed needed to escape Mars from the surface, about 5.03 km/s — less than half Earth's, which is why a return mission is even thinkable.

Escape Velocity of Mercury

v_{\mathrm{esc},\mathrm{Mercury}} = 4,250\ \text{m/s}

m/sSpeed needed to escape Mercury from its surface, about 4.25 km/s — too low to hold an atmosphere against 700 K daytime heat.

Escape Velocity of Neptune

v_{\mathrm{esc},\mathrm{Neptune}} = 23,500\ \text{m/s}

m/sSpeed needed to escape Neptune from the 1-bar level, 23.5 km/s — a well that even governs who stays put in the Kuiper belt beyond.

Escape Velocity of Saturn

v_{\mathrm{esc},\mathrm{Saturn}} = 35,500\ \text{m/s}

m/sSpeed needed to escape Saturn from the 1-bar level, 35.5 km/s — the well Cassini deliberately fell into at the end of its mission.

Escape Velocity of Uranus

v_{\mathrm{esc},\mathrm{Uranus}} = 21,300\ \text{m/s}

m/sSpeed needed to escape Uranus from the 1-bar level, 21.3 km/s — deep enough to keep hydrogen for the age of the solar system.

Escape Velocity of Venus

v_{\mathrm{esc},\mathrm{Venus}} = 10,360\ \text{m/s}

m/sSpeed needed to escape Venus from its surface, 10.36 km/s — nearly Earth's, which is why Venus kept a crushing atmosphere and its water did not survive anyway.

Jupiter Orbit Semi-Major Axis

a_{\mathrm{Jup}} = 7.7857 \times 10^{11}\ \text{m}

mMean distance of Jupiter from the Sun, 778.57 million km or 5.204 au — the orbit that organises the architecture of the solar system.

Mars Orbit Semi-Major Axis

a_{\mathrm{Mars}} = 2.2792 \times 10^{11}\ \text{m}

mMean distance of Mars from the Sun, 227.92 million km or 1.524 au — the orbit whose stubborn eccentricity taught Kepler the ellipse.

Mass of Jupiter measured

M_{\mathrm{J}} = 1.8982 \times 10^{27}\ \text{kg}

kgMass of Jupiter, 1.898 × 10²⁷ kg — 318 Earths, and more than twice all the other planets combined; the unit for weighing exoplanets.

Mass of Mars measured

M_{\mathrm{Mars}} = 6.4171 \times 10^{23}\ \text{kg}

kgMass of Mars, 6.417 × 10²³ kg — about 10.7 per cent of Earth's, small enough that the planet lost most of its atmosphere.

Mass of Mercury measured

M_{\mathrm{Mercury}} = 3.3011 \times 10^{23}\ \text{kg}

kgMass of Mercury, 3.301 × 10²³ kg — the smallest planet, yet the second densest, with an iron core filling most of its volume.

Mass of Neptune measured

M_{\mathrm{Neptune}} = 1.02413 \times 10^{26}\ \text{kg}

kgMass of Neptune, 1.024 × 10²⁶ kg — 17.1 Earths, the densest of the giant planets and the one found with mathematics before a telescope.

Mass of Saturn measured

M_{\mathrm{Saturn}} = 5.6834 \times 10^{26}\ \text{kg}

kgMass of Saturn, 5.683 × 10²⁶ kg — 95 Earths spread so thinly that its mean density, 687 kg/m³, is less than that of water.

Mass of Uranus measured

M_{\mathrm{Uranus}} = 8.681 \times 10^{25}\ \text{kg}

kgMass of Uranus, 8.681 × 10²⁵ kg — 14.5 Earths of hydrogen, helium and icy volatiles, tipped on its side at 98 degrees.

Mass of Venus measured

M_{\mathrm{Venus}} = 4.8675 \times 10^{24}\ \text{kg}

kgMass of Venus, 4.8675 × 10²⁴ kg — 81.5 per cent of Earth's, making it our closest twin in bulk and nothing like it in climate.

Mean Radius of Mars measured

R_{\mathrm{Mars}} = 3,389,500\ \text{m}

mVolumetric mean radius of Mars, 3389.5 km; the equatorial radius is 3396.2 km and the polar 3376.2 km, a 20 km flattening.

Mean Radius of Mercury measured

R_{\mathrm{Mercury}} = 2,439,700\ \text{m}

mVolumetric mean radius of Mercury, 2439.7 km — the smallest planet, barely a third of Earth's radius and still shrinking as its core cools.

Mean Radius of Venus measured

R_{\mathrm{Venus}} = 6,051,800\ \text{m}

mVolumetric mean radius of Venus, 6051.8 km — 95 per cent of Earth's, measured by radar through clouds no telescope can pierce.

Mercury Orbit Semi-Major Axis

a_{\mathrm{Mercury}} = 5.7909 \times 10^{10}\ \text{m}

mMean distance of Mercury from the Sun, 57.91 million km or 0.387 au — though its eccentric orbit swings 24 million km either side of it.

Neptune Orbit Semi-Major Axis

a_{\mathrm{Neptune}} = 4.49506 \times 10^{12}\ \text{m}

mMean distance of Neptune from the Sun, 4495.1 million km or 30.05 au — the outer edge of the planetary system, four light-hours out.

Polar Radius of Jupiter measured

b_{\mathrm{J}} = 66,854,000\ \text{m}

mJupiter's polar radius at the 1-bar level, 66 854 km — 4638 km less than the equatorial, squashed by a ten-hour rotation.

Polar Radius of Neptune measured

b_{\mathrm{Neptune}} = 24,341,000\ \text{m}

mNeptune's polar radius at the 1-bar level, 24 341 km — a 1.7 per cent flattening, the roundest figure among the four giant planets.

Polar Radius of Saturn measured

b_{\mathrm{Saturn}} = 54,364,000\ \text{m}

mSaturn's polar radius at the 1-bar level, 54 364 km — nearly 6000 km short of the equator, the most flattened planet there is.

Polar Radius of Uranus measured

b_{\mathrm{Uranus}} = 24,973,000\ \text{m}

mUranus's polar radius at the 1-bar level, 24 973 km — a 2.3 per cent flattening, with the odd twist that a pole often faces the Sun.

Saturn Orbit Semi-Major Axis

a_{\mathrm{Saturn}} = 1.43353 \times 10^{12}\ \text{m}

mMean distance of Saturn from the Sun, 1433.5 million km or 9.58 au — the edge of the known solar system for all of human history until 1781.

Sidereal Orbital Period of Jupiter

T_{\mathrm{J}} = 374,335,689.6\ \text{s}

sJupiter's year, 4332.589 days or 11.86 Earth years — close enough to twelve that it paced calendars across the ancient world.

Sidereal Orbital Period of Mars

T_{\mathrm{Mars}} = 59,355,072\ \text{s}

sThe Martian year, 686.980 days or 1.881 Earth years — the beat that sets the 26-month rhythm of every launch window.

Sidereal Orbital Period of Mercury

T_{\mathrm{Mercury}} = 7,600,521.6\ \text{s}

sMercury's year, 87.969 days — the fastest orbit of any planet, which earned it the name of the Roman gods' winged messenger.

Sidereal Orbital Period of Neptune

T_{\mathrm{Neptune}} = 5,200,329,600\ \text{s}

sNeptune's year, 60 189 days or 164.8 Earth years — it completed its first full orbit since discovery on 11 July 2011.

Sidereal Orbital Period of Saturn

T_{\mathrm{Saturn}} = 929,596,608\ \text{s}

sSaturn's year, 10 759.22 days — 29.46 Earth years, the generation-long cycle that made it antiquity's planet of old age and time.

Sidereal Orbital Period of Uranus

T_{\mathrm{Uranus}} = 2,651,218,560\ \text{s}

sUranus's year, 30 685.4 days — 84 Earth years, so the sideways planet gives each pole a 42-year day and a 42-year night.

Sidereal Orbital Period of Venus

T_{\mathrm{Venus}} = 19,414,166.4\ \text{s}

sVenus's year, 224.701 days — remarkable chiefly because it is shorter than the planet's own 243-day rotation.

Sidereal Rotation Period of Jupiter

T_{\mathrm{rot},\mathrm{J}} = 35,730\ \text{s}

sOne rotation of Jupiter takes 9 h 55.5 min — the fastest spin of any planet, defined by the magnetic field because the clouds disagree.

Sidereal Rotation Period of Mars

T_{\mathrm{rot},\mathrm{Mars}} = 88,642.44\ \text{s}

sOne true rotation of Mars, 24.6229 hours — the near-match to Earth's day that gives rover teams their slightly drifting 'sol'.

Sidereal Rotation Period of Mercury

T_{\mathrm{rot},\mathrm{Mercury}} = 5,067,360\ \text{s}

sMercury's true rotation period, 58.646 days — exactly two thirds of its year, a 3:2 resonance discovered by radar in 1965.

Sidereal Rotation Period of Neptune

T_{\mathrm{rot},\mathrm{Neptune}} = 57,996\ \text{s}

sOne rotation of Neptune takes 16.11 hours, clocked from Voyager 2's radio data during the single close encounter ever made.

Sidereal Rotation Period of Saturn

T_{\mathrm{rot},\mathrm{Saturn}} = 38,361.6\ \text{s}

sSaturn's rotation, conventionally 10.656 h from Voyager's radio data — a genuinely uncertain number on a planet that hides its own clock.

Sidereal Rotation Period of Uranus

T_{\mathrm{rot},\mathrm{Uranus}} = 62,064\ \text{s}

sOne rotation of Uranus takes 17.24 hours, retrograde by the tilt's own geometry — a Voyager radio measurement never since repeated.

Sidereal Rotation Period of Venus

T_{\mathrm{rot},\mathrm{Venus}} = 20,997,360\ \text{s}

sOne rotation of Venus takes 243.02 days, and it turns backwards — the slowest and the only retrograde spin among the inner planets.

Surface Gravity of Jupiter measured

g_{\mathrm{J}} = 24.79\ \text{m/s}^{2}

m/s²Gravitational acceleration at Jupiter's 1-bar level on the equator, 24.79 m/s² — 2.5 times Earth's, before the fast spin refunds part of it.

Surface Gravity of Mars measured

g_{\mathrm{Mars}} = 3.71\ \text{m/s}^{2}

m/s²Equatorial surface gravity on Mars, 3.71 m/s² — 38 per cent of Earth's, the figure every Mars lander design is built around.

Surface Gravity of Mercury measured

g_{\mathrm{Mercury}} = 3.7\ \text{m/s}^{2}

m/s²Gravitational acceleration at Mercury's surface, 3.70 m/s² — almost exactly the same as Mars, on a planet half the diameter.

Surface Gravity of Neptune measured

g_{\mathrm{Neptune}} = 11.15\ \text{m/s}^{2}

m/s²Gravitational acceleration at Neptune's 1-bar equator, 11.15 m/s² — the only planet besides Jupiter that out-pulls the Earth.

Surface Gravity of Saturn measured

g_{\mathrm{Saturn}} = 10.44\ \text{m/s}^{2}

m/s²Gravitational acceleration at Saturn's 1-bar equator, 10.44 m/s² — 95 Earth masses producing barely more pull than Earth itself.

Surface Gravity of Uranus measured

g_{\mathrm{Uranus}} = 8.69\ \text{m/s}^{2}

m/s²Effective gravity at Uranus's 1-bar equator, 8.69 m/s² — including the spin's centrifugal refund; gravity alone would be 8.87.

Surface Gravity of Venus measured

g_{\mathrm{Venus}} = 8.87\ \text{m/s}^{2}

m/s²Gravitational acceleration at the surface of Venus, 8.87 m/s² — 90 per cent of Earth's, the most Earth-like gravity of any planet.

Uranus Orbit Semi-Major Axis

a_{\mathrm{Uranus}} = 2.87246 \times 10^{12}\ \text{m}

mMean distance of Uranus from the Sun, 2872.5 million km or 19.2 au — the discovery that doubled the solar system overnight in 1781.

Venus Orbit Semi-Major Axis

a_{\mathrm{Venus}} = 1.0821 \times 10^{11}\ \text{m}

mMean distance of Venus from the Sun, 108.21 million km or 0.723 au, on the most nearly circular orbit of any planet.